Measuring particle size can be more complicated than one would think. If all particle were spherical in shape, one could simply measure the diameter or the radius, and thereby get the correct size. However, sence particles are rarely perfectly spherical in shape, the concept of "size" becomes more difficult.

The term "equivalent spherical size" are often used. This means that the "diameter" assigned to an irregularly shaped parictle is the same as that of a sphere which behaves identically in the same process.

There are several different techniques utilised to measure particle size. Some take shape into account, whereas others don't. All methods have their benefits and drawbacks, so choosing the optimal method for each sample or application is therefore important.